1. Field of the Invention
The present invention relates to a sensor for detecting a high molecular weight substance present in a low molecular weight gas such as air, and more particularly to a sensor which can be used in environmental and medical measurements or semiconductor manufacturing processes for monitoring a trace amount of a harmful high molecular weight substance present in air or other low molecular weight gas, the sensor being highly sensitive, compact and lightweight and being convenient for portable use.
2. Description of the Prior Art
An example of an apparatuses used for detecting high molecular weight substances present in the air is disclosed in U.S. Pat. No. 4,769,540. The apparatus disclosed in this patent is based on the atmospheric ionization mass spectrometric method. This method comprises the steps of ionizing molecules of the air to yield light ions, ionizing a polymeric substance present in the air through an ion-molecule reaction between the light ions and the high molecular weight substance to thus yield heavy ions, and then guiding these light ions and heavy ions to a mass spectrometer to quantitatively analyze the heavy ions. In this first conventional apparatus, however, the path through which these ions are transported must be maintained at a high vacuum in order to separate the ions from each other and obtain correct mass spectra. Since a high vacuum pump is needed, such an apparatus cannot be installed at any desired place, and furthermore carrying the apparatus and using it for onsite measurements is cumbersome.
Another known method comprises the steps of forming light and heavy ions as in the foregoing conventional apparatus, extracting ions by applying a pulse voltage to a grid disposed perpendicularly to the direction of flow of the air containing these ions, and separating and determining the heavy ions according to the atmospheric ionization flight time measuring method. This method makes use of the difference in time for the light and heavy ions to arrive at the grid after the application of the pulse voltage, since the flight velocity of the light ions differs from the that of the heavy ions. In this method, however, the effective flight time separation of masses is ensured only when the ionization chamber is as small as possible so as to restrict, to a narrow region, the flight-initiation position of the ions attracted to the grid through the application of the pulse voltage. For this reason, it is necessary to use only radiation emitted toward a specific direction obtained through the collimation of the radiation emitted from a source such as .sup.63 Ni, which emits radiation having a short trajectory. This greatly restricts the resulting ionization efficiency. In addition, the effective time which is available for the measurement is greatly limited by the time interval of the pulse-voltage application. Furthermore the use of a source (e.g., .sup.63 Ni, 50 mCi) which emits very strong radiation would be severely restricted by regulations on the handling of radioactive substances.
GB 2,228,139A discloses an improved ion-mobility-detector based on the atmospheric ionization flight time measuring method. This detector comprises an ion source disposed at a position branching off from a ion reaction region, at least two drift regions which communicate with the ion reaction region, an electrical ion injection shutter disposed between the ion reaction region and each drift region, and an ion detector positioned at the end of each drift region far apart from the ion reaction region. In each drift region, a potential gradient is established so as to define the drift direction of the ions. The detection of ions in a sample gas is performed on the basis of the lapse of time from the opening of the ion injection shutters till the ions are detected by the ion detectors, or on the basis potential applied to the ion source and the timing of an opening pulse applied to the ion injection shutters is adjustable. This apparatus makes it possible to perform separation and detection or quantitative analysis for every drift mobility or for approximately every ionic species, but to this end, it is necessary to adjust the timing of the application of a potential to the ion source and the timing of the application of an opening pulse to the ion injection shutters for every ionic species, and a high speed-high resolution pulse measuring circuit must be used.
Further, U.S. Pat. Nos. 3,835,328 and 4,238,678 disclose other apparatuses for detecting trace amounts of impurity gas components in gaseous samples operated at atmospheric pressure.
The apparatus disclosed in U.S. Pat. No. 3,835,328 is mainly provided with a recombination region positioned downstream from an ionization region which is equipped with a radiation source. This recombination region comprises electrically insulating washers fitted into a shaft at proper intervals, each having a hole of a different diameter to form a tortuous path through which the impurity gas ions can pass. The impurity ions pass through the tortuous path while the recombination of the ionic main components of the sample gas is promoted during transfer therethrough. The ions are detected by a screen of an electrically conducing material in a collection region positioned downstream from the recombination region. The shaft is electrically connected to the conductive screen through a DC voltage source and an ammeter arranged in series, and a DC voltage is applied between the shaft and the screen to permit an electric current which corresponds to the concentration of the impurity to be read out.
The detecting apparatus disclosed in U.S. Pat. No. 4,238,678 comprises a first drift region which is defined by a grid and a shutter and which is positioned downstream from an ionization region, and a second drift region which is defined by the shutter and a collector electrode and which is positioned downstream from the shutter. The grid comprises a plurality of conducting materials arranged parallel to one another and is positioned perpendicularly to the flow direction of a sample gas stream. An electric voltage is alternatively applied to the plurality of conducting materials so that each pair of neighboring conducting materials have different potentials at each instance. Thus, the grid serves to collect light ions having a high drift mobility among the ions present and to conduct heavy ions having a low drift mobility to the first drift region. Moreover, the shutter has the same structure and arrangement as the grid and serves to guide specific ions or discrete packets of ions among the low mobility ions which drift through the first drift region to the second drift region. A conductive ring is arranged around the periphery of the first and second drift regions, through which an electric field is established. The field has a desired potential gradient which is parallel to the direction of the sample gas stream containing ions. The drift velocities of ions in the first and second drift regions depend on the foregoing electric field and the ions which drift through the second drift region are collected and detected by a collector electrode positioned at the downstream end of the second drift region.
U.S. Pat. No. 4,119,851 discloses an apparatus for detecting impurities in a gaseous sample which comprises a recombination tube as disclosed in U.S. Pat. No. 3,835,328 and a drift tube as disclosed in U.S. Pat. No. 4,238,678 which are connected in series.
The recombination tube disclosed in U.S. Pat. No. 3,835,328 allows the recombination of main ionic components of a sample gas as soon as possible to remove the same and thus serves to guide only the impurity ions to a detector, while the drift tube disclosed in U.S. Pat. No. 4,238,678 serves to separate and detect impurity ions for all drift mobilities, approximately corresponding to their molecular weights, or for every species of the impurities.
As has been discussed above in detail, all of the conventional apparatuses for detecting impurities in a gas sample are designed in such a manner that only impurity ions are guided to detectors such as collector electrodes. In these apparatuses, it is sufficient that only a part of the impurity ions are involved in the detection, but the ions of the main components of the gas obstruct the detection of the impurity ions.
Furthermore, the grid and shutter disclosed in U.S. Pat. No. 4,238,678 are both realized by arranging a plurality of conductive materials perpendicularly to the flow direction of a sample gas containing ions so that there does not occur any warping in the distance between neighboring pairs of conductive materials and so that a desired electric field can thus be established therebetween. This makes the production of these apparatuses very difficult.